Various sensor systems have been developed for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Sensors for continuous flat sheet production processes typically employ single or dual-sided packages with on-line sensors that traverse or scan traveling webs of sheet material during manufacture. With dual scanners, the heads or assemblies are fixed to beams that span both sides of the sheet with linear guidance tracks to allow the sensors to move in unison in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet-making operation, cross-directional distances can range up to about twelve meters or more. In the paper making art, for instance, the on-line sensors detect variables such as basis weight, moisture content, and caliper of sheets during manufacture.
On-line measurements are difficult to make accurately. In the case where the sensor comprises dual scanner heads, under normal operating conditions the alignment of the upper and lower track systems are similar to factory alignment. However, if the mill-operating environment exposes the beams to uneven thermal loading, the upper and lower sensor paths will deviate from factory specifications. This will cause variations in sensor readings across the width of the sheet.